Transformer load indicator



May 24, '1938. L. WETHERILL 2,118,560

TRANSFORMER LOAD INDICATOR Filed Sept. 10, 1936 Invent 0v La Lgnn \XfethePill Ambient Ymper'ature (0.) His Attovneg.

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UNITED STATES PATENT orr ca TRANSFORMER LOAD INDICATOR Lynn W'etherlll, Pittsfield, Masa, assignor to Geneyralk Electric Company, a corporation of New Application September 10, 1938, Serial No. 100,159 7 Claims. (Cl. 78-85.)

Myinvention relates to transformer load indlthe wall of a casing ll of a transformer imcators. As the load on a transformer increases, mersed in a specially prepared insulating oil II the internal losses increase. These losses appear or other suitable insulating liquid with the bulb as heat and -if the load becomes too great, it It immersed in the liquid l8 near its top surface 5 may cause such internal temperatures as may within the casing I1 and the instrument casing 5 cause damage to the transformer. The maxi- II with its enclosed parts suspended in the air mumload which a transformer can carry safely outside the casing l I. The instrument casing is not always the same but varies with the rate II also encloses a thermostat is which is therat which the internal heat can be dissipated. mally responsive to the temperature of the air 10 Thus, a greater load can be safely carried when outside the casing H or, in other words-to the 10 the air surrounding the transformer is cold than ambient temperature. The Bourdon tube It is when it is warm because the heat from the responsive to the temperature of the top oil it. transformer will be dissipated more rapidly to An arm or lever 20 is secured to a shaft conthe cold air. For this reason, an indication of trolled by the thermostat l9 and a similar arm the load which a transformer is carrying at any or lever 2| is secured to a shaft controlled by 15 time is often, little indication of whether or the Bourdon tube i4. One end of a rod or link not the transformer is overloaded. However, 22 is pivotally connected to the outer end of instruments have been proposedand used which the lever 20. One end of a similar rod or link indicate what percentage of the maximum safe .23 is connected to the outer end of the lever load the transformer is carrying and, with this 2|. The other ends of the two links 22 and 23 2o latter indication, it is easy'to determine at any are connected together bya pivotal connection 24. time whether the load on a transformer can be As shown diagrammatically in Fig. 4,. a link safely increased and if so, how much more load with a pivotal support 28 at one end is conthe transformer can safely carry. nected by a link 21 to the pivotal connection 25 The general object of the present invention 24 between the links 22 and 23, the link 25 and $5 is to provide an improved instrument for indithe link 21 being connected together by a pivotal eating with substantial accuracy under any temconnection 28. The link 25 is extended beyond perature and load conditions the ratio of the the connection 28 to form a pointer 28 movable load being carried by a transformer to the maxiover a scale I0.

mum safe load which the transformer could carry If the parts which have been so far described so under the same temperature conditions. are properly proportioned, then the pointer 29 The invention will be better understood from will indicate at any time on the scale 30 what the following description taken in connection proportion of the maximum safe load the transwlth the accompanying drawing in which Fig. 1. former is carrying. Thus, in Fig. 4, the pointer is a view of a load indicating instrument con- 29 is indicating the value 0.5 which means that 35 structed in accordance with the invention, parts the transformer is carrying 50% of the maximum being broken away to show details; Fig. 2 is a load which it might carry safely under the same sectional view of the instrument on the line 2-4 ambient temperature conditions. The necessary of Fig. 1; Fig. 3 is a side view of the instrument proportions and relations of the various parts 40 assembled witha supporting structure arranged of the instrument have not been calculated 40 for suspension over the upper edge of the casing mathematically as the calculations would be exof a transformer; Fig. 4 is an explanatory diatremely complicated, but it has been found by grammatic view of the instrument: Fig. 5 is an very careful tests that an instrument will give explanatory curve diagram. very accurate indications when designed graphi- 46 Like reference characters indicate similar parts cally in accordance with the following explain the different figures of the drawing. nation.

The preferred form of the instrument which The position of the pivotal connection 24 is forms the subject matter of the present invention controlled by the angular positions of the levers is shown most clearly in Figs. 1 and 2. This 20 and 2| and in turn determines the position 50 instrument ll includes a casing ll withaface of the pointer 29 on the scale 30. The thermo- 50 I2 covered by a transparent glass It to exclude stat is and-the Bourbon tube It are preferably dust. The casing ll encloses a Bourdon tube arranged to impart angular movementsof about I! connected by a capillary tube I! to a bulb to to their respective levers 20 and 2| ll. The instrument III is mounted on a frame between the extreme positions corresponding to so arranged for suspension over the upper edge of the maximum variations to be expected in the 56 right angle to each other when temperatures of the air outside the transformer casing i1 and of the insulating liquid II. The levers 23 and 2i should be at approximately a the ambient and liquid temperatures which control them are about midway between their maximum and minimum valuu. With the levers 23 and 2| in these relative positions. the links 22 and 23 should be so proportioned that the angle between them will be about a right angle and they should be of about the same length.

The temperature of a transformer increases with its load. and the maximum load which a transformer can safely carry is determined by the maximum temperature which will not injure the transformer. The top oil temperature depends upon the ambient temperature and the load which the transformer is carrying. This top oil temperature can be determined for any given load and ambient temperature conditions either by testing an actual transformer or by methods of calculation well known to transformer engineers. The results may be shown in the form of curves. such as the curves 3|, 32, 33, and 34 L of Fig. 5. The curve 3| indicates the top oil temperatures corresponding to difl'erent ambient temperatures when the transformer is carrying its maximum safe load. The curve 32 indicates the top oil temperatures corresponding to different ambient temperatures when the transformer is carrying 75% of the maximum load which it could carry under the different ambient temperature conditions. The curve 33 indicates the top oil temperatures corresponding to different ambient temperatures when the transformer is carrying 50% of the maximum safe load which it could carry under the different ambient temperature conditions. The curve 34 indicates the top oil temperatures corresponding to different ambient temperatures when the transformer is carrying no load, but assuming that the transformer is excited and assuming that the excitation of the transformer will increase the temperature of the oil 10 above any ambient temperature. It is obvious that these curves 3| to 34 inclusive will meet in a single point where the top oil temperature is 10 above the ambient temperature. The position of the pivotal connection 24 is controlled by the two levers 23 and 2| and moves in response to changes in either the ambient temperature or the top oil temperature or both. For any given ambient temperature the position of the lever 23 is fixed and the pivotal connection 24 may be moved by the lever 2| along an arc of a circle, and arcs of circles may thus be drawn to correspond to different ambient temperatures. In Fig. 4 the position of the lever 23 corresponds to an ambient temperature of 20. In a similar manner, arcs of circles may be drawn for different positions of the lever 2| corresponding to different top oil temperatures. These two families of arcs constitute a coordinate' system between ambient and top oil temperatures corresponding to the coordinate system shown in Fig. 5, except that the coordinates of the system shown in Fig. 4 are curvilinear while thoseoi' Fig. 5 are rectilinear. The curves 3|, 32, 33, and 34 of Fig. 5 may now be transferred to the curvilinear coordinate system of Fig. 4 where they will appear of course as curved lines rather than as substantially straight lines. It will be found that each of the curves 3i, 32, 33, and 34 when transferred to the curvilinear system of Fig. 4 will beat least very nearly an arc of a true circle, that these arcs will of course intersect in a single point, and that all of these arcs have radii of very nearly the same length. If the curves 3|, 32, 33, and 34 differ at all from arcs of true circles having the same radii; then they should be changed slightly to meet these conditions and it is considered preferable that the arc of the circle be selected which most nearly fits the curve 3! because the greatest instrument accuracy is usually desirable for maximum safe load conditions rather than for conditions under which the transformer is only partially loaded. The centers 3f, 32', 33' and 34' of the curves 3i, 32, 33, and 34 are now found asindicated in Fig. 4 and an arc 33 of a circle is drawn through these centers. The pivotal support 23 of the link 23 should be located at the center of the arm 33 which will be at the intersection of the curves 3i, 32, 33, and 34. The link 21 is disposed in the angle between the links 22 and 23 and should be equal to the r'adiiof the curves 3i, 32, 33 and 34 and therefore to the length of the link 23 between its pivotal support 23 and the pivotal connection 23 so that the pivotal connection 23 between the links 23 and 21 will always be somewhere on the are 33, depending upon the position of the pivotal connection 24.

The curves 3|, 32, 33, and 34 on the curvilinear coordinate diagram of Fig. 4 form a chart upon which the position of the pivotal connection 24 will indicate at any time the proportion or per-,

centage of the maximum safe load which the transformer is. carrying. Additional load curves may of course be added to the chart if desired so that the indication may be more easily read. This form of chart, however, may not be as desirable as a linear scale provided with a pointer and such a linear chart or scale 33 is made possible by the addition of the links 23 and 21 connected and proportioned as has already been described. The indications on the scale 33 and on the curvilinear coordinate chart are identical.

In the preferred form of the invention shown in Fig. l, the pointer 23 of Fig. 4 is omitted and a pointer 33 secured at one end to a pivoted stud 31 is movable over a linear chart or scale 33. The pointer 33 is provided with a pin 33 in the path of a cam 40 carried by the link 23. The distance between the pin 33 and the pivot 31 is relatively small as compared with the length of the pointer 33 so that any angular movement of the link 23 produces a considerably greater angular movement of the pointer 33. The graduations of the scale 33 are therefore more widely spaced than the corresponding graduations of the scale 33 of Fig. 4 and they may therefore be read more easily. The pointer 33 is of the maximum reading type, that is, the link 23 will move the pointer 36 in a clockwise direction but this pointer will be held in its maximum position by a stop 4i engaging a roughened or toothed surface of a member 42 secured to the pivot 31 of the pointer. The stop 4| is mounted on a shaft 4i provided with a handle 43 extending through an opening in the casing l i. By raising this handle 43 the stop 4| will be disengaged from the member 42 to allow the pointer 33 to be returned by a weight 44 to the zero position on the scale 33 or to a position in which the pin 33 strikes the cam 43. A vane 45 rotatable with the pointer 33 functions as a danger signal. This vane may be concealed under normal conditions but may be arranged to appear through a window in the casing when the reading of the pointer 33 approaches a danger point on the scale 33.

The maximum safe load which a transformer 75 auaeoo can carry is limited by the maximum safe temperature of the transformer. This temperature increases with the load and also with the ambient temperature. A greater load can be carried with safety therefore under low ambient temperature conditions than under high ambient temperature conditions. A simple indication of the load itself is therefore not very helpful in determining what proportion or percentage of the maximum safe load a transformer may be carrying. The present invention, however, provides an instrument which shows directly at any time and under any ambient temperature conditions whether a transformer is fully loaded or overloaded, or, if underloaded, then the relation between the load being carried by the transformer and the maximum saie load which the transformer could carry under the same liquid and air temperature conditions. The instrument also indicates how much more load it can safely carry.

The invention has been explained by describing useful forms thereof but it is obvious that changes may be made without departing from the spirit of the invention and the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An indicating instrument including means ior response to temperature changes in the cooling liquid of liquid-immersed electrical apparatus, means for response to temperature changes in the air surrounding the apparatus, a link controlled by each temperature-responsive means, a pivotal connection between the links, a load chart, and indicating means controlled by the links, said chart and the lengths and angular relations of said links being proportioned to indicate the relation between the load carried by the apparatus and the maximum safe load under the same liquid and air temperature conditions.

2. An indicating instrument including means for response to temperature changes in the cooling liquid of liquid-immersed electrical apparatus, means for response to temperature changes in the air surrounding the apparatus, a link controlled'by each temperature-responsive means, a pivotal connection between the links, a load chart, and indicating means movable over said chart and pivotally connected to said pivotal connection, said chart and the lengths and angular relations of said links being proportioned to indicate the relation between the load carried by the apparatus and the maximum safe load under the same liquid and air temperature conditions.

3. An indicating instrument including two levers, means for controlling one of said levers in response to temperature changes in the cooling liquid of liquid-immersed electrical apparatus, means for controlling the other lever in response to temperature changes in the air surrounding the apparatus. a link pivotally connected at one end to one lever, a second link pivotally connected at one end to the other lever, a pivotal connection between the other ends of said links, a load chart, and indicating means controlled by said links, said chart and the lengths and angular relations oi. said levers and links being proportioned to indicate the relation between the load carried by the apparatus and the maximum safe load under the same liquid and air temperature conditions.

4. An indicating instrument including two levers, means for controlling one of said levers in response to temperature changes in the cooling liquid of liquid-immersed electrical apparatus, means for controlling the other lever in response to temperature changes in the air surrounding the apparatus, a link pivotally connected at one end to one lever, a second link pivotally connected at one end to the other lever, a pivotal connection between the other ends of said links, a load chart, and indicating means connected to the pivotal connection between'said links, said chart and the lengths and angular relations of said levers and links being proportioned to indicate the relation between the load carried by the apparatus and the maximum safe load under the same liquid and air temperature conditions.

5. An indicating instrument including two levers, means for controlling one of said levers in response to temperature changes in the cooling liquid of liquid-immersed electrical apparatus, means for controlling the other lever in response to temperature changes in the air surrounding the apparatus, said levers being disposed at approximately a right angle to each other durin average temperature conditions of the liquid and air, a link pivotally connected at one end to one lever, a second link pivotally connected at one end to the other lever, a pivotal connection between the other ends of said links, said links being disposed at approximately a right angle to each other during average temperature conditions of the liquid and air, a load chart, a pointer movable over said load chart, and a link connecting said pointer and said pivotal connection,

said latter link vbeing disposed in the angle between said first two links, and said chart and the lengths and angular relations of said levers and links being proportioned to indicate the relation between the load carried by the apparatus and the maximum safe load under the same liquid and air temperature conditions.

. LYNN WETHERIIL. 

